What are the implications for a woman who vowed to fast perpetually "every Monday and Thursday, and the middle days of the Arabic months" because she was unable to fast during Ramadan, and is now forty years old?

You must make up for the days you missed in Ramadan due to a valid excuse. If you delayed making them up until another Ramadan began, then you owe expiation: feeding one poor person for each day. Voluntary fasting, such as Mondays and Thursdays, and three days of each month, does not suffice for making up Ramadan fasts unless you intended them as make-up fasts. If your statement, "O Lord, I will fast Mondays and Thursdays and the middle days of the month" was a vow, then you are obligated to fulfill it unless you become unable to do so, in which case you owe expiation for breaking an oath.